This invention relates to lens-exchangeable cameras of the TTL (through the lens) light measurement type, capable of selecting dual exposure control modes, and more particularly relates to a locking device for such cameras, which device can lock a diaphragm presetting member of an exchangeable lens at a specific position wherein indication of a f-number representative of an automatically controlled diaphragm aperture is allowed with the diaphragm being held at its fully open aperture.
The term "dual exposure control mode" as used herein relates to modes of photography of a camera, in which a shutter speed priority diaphragm automatic control mode and at least either a diaphragm priority shutter speed automatic control mode or a so-called manual exposure control mode, in which both a shutter speed and a diaphragm aperture are manually set for exposure, may be changed over from one to the other.
In order for cameras of the above described type to enable indication of a f-number representative of an automatically controlled diaphragm aperture with the diaphragm of an exchangeable lens being held at its fully open aperture, information of the fully open aperture inherent to each exchangeable lens is necessary. To this end, in the case of the shutter speed priority diaphragm automatic control mode, a diaphragm presetting member of an exchangeable lens mounted on a camera body is set to a specific position wherein information of the fully open aperture of the exchangeable lens can be directly or indirectly introduced into an indicating device within the camera body.
Moreover, photography is often repeatedly taken in the shutter speed priority diaphragm automatic control mode in cameras of the above described type. In this case, it is necessary that the diaphragm presetting member remains in the specific position as described. However, it is mandatory that the diaphragm presetting member is prevented from being moved from the specific position even with inadvertent contact of a photographer's hand or the like. Otherwise, the photography intended by the photographer can not be continuously taken since the camera can no longer indicate that f-number representative of an automatically controlled diaphragm aperture.